The significance of polycationic proteins was recently brought to focus by a "First International Conference" held in Brussels, Oct. 1982 on the "Pathogenic Role of Cationic Proteins' Interaction with Biological Membranes". While glomerular fixed negative charge neutralization has been implicated in pathogenetic mechanisms of proteinuria, observations that tight junctions are also induced by cationic proteins in glomerular epithelial cells have received little attention. It is our contention that in leaky epithelia tight junctions act as a dynamic partial permeability barrier between lumen (mucosa) and intercellular space and are regulated by cytoplasmic mechanisms. One trigger for "induction" of tight junctions appears to be neutralization of fixed negative charge on the surface of the membrane domain adjacent to the tight junction. To understand this phenomena, protamine (5,000 daltons, pI 9.7 to 12) will be applied to the mucosal surface of Necturus gallbladder mounted in vitro and electrophysiological responses will be correlated with morphologic changes in the intramembranous appearance of the tight junctional meshwork of fibrils (freeze fracture electronmicroscopy). To better interpret changes in transjunctional conductance, cellular activity of K+ and Cl- ions will be measured by ion-sensitive microelectrodes and changes in membrane conductance will be assessed by ion substitution experiments. In this way, we hope to identify a mucosal concentration of protamine that will alter junctional conductance but have a minimal effect on the transcellular path. The comparatively large change in junctional conductance induced by protamine will be utilized to define the structural counterpart which correlates best with ionic conductance. "Blister" formation within the junctional fibrillar meshwork induced by mucosa to serosa osmotic water flux may provide additional insight into this question. Since it seems unlikely that cationic proteins react directly with junctional elements, we will test several hypotheses: stimulation of intracellular cAMP and loss of microfilament-membrane attachment visualized ultrastructurally. Ultimately, the significance of the "leaky" tight junction might be appreciated through understanding mechanisms which regulate tight junctional permeability and their potential modulation by volume flow in the intercellular space resulting from active transcellular salt and water transport.